Lean burn internal combustion engine exhaust gas temperature control

ABSTRACT

A variety of methods and arrangements for controlling the exhaust gas temperature of a lean burn, skip fire controlled internal combustion engine are described. In one aspect, an engine controller includes an aftertreatment system monitor and a firing timing determination unit. The aftertreatment monitor obtains data relating to a temperature of one or more aftertreatment elements, such as a catalytic converter. Based at least partly on this data, the firing timing determination unit generates a firing sequence for operating the engine in a skip fire manner such that the temperature of the aftertreatment element is controlled within its effective operating range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/654,373 filed Oct. 16, 2019, which is Continuation of U.S.application Ser. No. 16/275,881 filed Feb. 14, 2019 (now U.S. Pat. No.10,494,971, issued Dec. 3, 2019), which is a Continuation of U.S.application Ser. No. 15/347,562, filed on Nov. 9, 2016 (now U.S. Pat.No. 10,247,072, issued Apr. 2, 2019), which claims priority to U.S.Provisional Application No. 62/254,049, filed on Nov. 11, 2015. All ofthe above listed applications are incorporated by reference herein intheir entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a skip fire engine control system foran internal combustion engine. More specifically, the present inventionrelates to arrangements and methods for controlling exhaust gastemperature to improve efficacy of an emission control system.

BACKGROUND

Most vehicles in operation today are powered by internal combustion (IC)engines. Internal combustion engines typically have multiple cylindersor other working chambers where combustion occurs. The power generatedby the engine depends on the amount of fuel and air that is delivered toeach working chamber. The engine must be operated over a wide range ofoperating speeds and torque output loads to accommodate the needs ofeveryday driving.

There are two basic types of IC engines; spark ignition engines andcompression ignition engines. In the former combustion is initiated by aspark and in the latter by a temperature increase associated withcompressing a working chamber charge. Compression ignition engines maybe further classified as stratified charge compression ignition engines(e.g., most conventional Diesel engines, and abbreviated as SCCI),premixed charge compression ignition (PCCI), reactivity controlledcompression ignition (RCCI), gasoline compression ignition engines (GCIor GCIE), and homogeneous charge compression ignition (HCCI). Some,particularly older, Diesel engines generally do not use a throttle tocontrol air flow into the engine. Spark ignition engines are generallyoperated with a stoichiometric fuel/air ratio and have their outputtorque controlled by controlling the mass air charge (MAC) in a workingchamber. Mass air charge is generally controlled using a throttle toreduce the intake manifold absolute pressure (MAP). Compression ignitionengines typically control the engine output by controlling the amount offuel injected (hence changing the air/fuel stoichiometry), not air flowthrough the engine. Engine output torque is reduced by adding less fuelto the air entering the working chamber, i.e. running the engine leaner.For example, a Diesel engine may typically operate with air/fuel ratiosof 20 to 55 compared to a stoichiometric air/fuel ratio of approximately14.5.

Fuel efficiency of internal combustion engines can be substantiallyimproved by varying the engine displacement. This allows for the fulltorque to be available when required, yet can significantly reducepumping losses and improve thermal efficiency by using a smallerdisplacement when full torque is not required. The most common methodtoday of implementing a variable displacement engine is to deactivate agroup of cylinders substantially simultaneously. In this approach theintake and exhaust valves associated with the deactivated cylinders arekept closed and no fuel is injected when it is desired to skip acombustion event. For example, an 8 cylinder variable displacementengine may deactivate half of the cylinders (i.e. 4 cylinders) so thatit is operating using only the remaining 4 cylinders. Commerciallyavailable variable displacement engines available today typicallysupport only two or at most three displacements.

Another engine control approach that varies the effective displacementof an engine is referred to as “skip fire” engine control. In general,skip fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, aparticular cylinder may be fired during one engine cycle and thenskipped during the next engine cycle and selectively skipped or firedduring the next. In this manner, even finer control of the effectiveengine displacement is possible. For example, firing every thirdcylinder in a 4 cylinder engine would provide an effective reduction to⅓^(rd) of the full engine displacement, which is a fractionaldisplacement that is not obtainable by simply deactivating a set ofcylinders to create an even firing pattern.

Both spark ignition and compression ignition engines require emissioncontrol systems including one or more aftertreatment elements to limitemission of undesirable pollutants that are combustion byproducts.Catalytic converters and particulate filters are two commonaftertreatment elements. Spark ignition engines generally use a 3-waycatalyst that both oxidizes unburned hydrocarbons and carbon monoxideand reduces nitrous oxides (NO_(x)). These catalysts require that onaverage the engine combustions be at or near a stoichiometric air/fuelratio, so that both oxidation and reduction reactions can occur in thecatalytic converter. Since compression ignition engines generally runlean, they cannot rely solely on a conventional 3-way catalyst to meetemissions regulations. Instead they use another type of aftertreatmentdevice to reduce NO_(x) emissions. These aftertreatment devices may usecatalysts, lean NO_(x) traps, and selective catalyst reduction (SCR) toreduce nitrous oxides to molecular nitrogen. The most common SCR systemadds a urea/water mixture to the engine exhaust prior to the engineexhaust flowing through a SCR based catalytic converter. In the SCRelement the urea breaks down into ammonia, which reacts with nitrousoxides in the SCR to form molecular nitrogen (N₂) and water (H₂O).Additionally, Diesel engines often require a particulate filter toreduce soot emissions.

To successfully limit engine emissions all aftertreatment systemelements need to operate in a certain elevated temperature range tofunction more efficiently. Since 3-way catalysts are used in sparkignition engines where the engine air flow is controlled, it isrelatively easy to maintain a sufficiently elevated engine exhausttemperature, in the range of 400 C, to facilitate efficient pollutantremoval in a 3-way catalyst. Maintaining adequate exhaust gastemperature in a lean burn engine is more difficult, since exhausttemperatures are reduced by excess air flowing through the engine. Thereis a need for improved methods and apparatus capable of controlling theexhaust gas temperature of a lean burn engine over a wide range ofengine operating conditions.

SUMMARY OF THE INVENTION

A variety of methods and arrangements for heating or controlling atemperature of an aftertreatment element in an exhaust system of a leanburn internal combustion engine are described. In one aspect, an enginecontroller includes an aftertreatment element monitor and a firingtiming determination unit. The aftertreatment element monitor isarranged to obtain data relating to a temperature of one or moreaftertreatment elements, such as a catalytic converter. This data may bein the form of an aftertreatment element temperature model and/or mayinvolve a direct measurement or sensing of the temperature of theaftertreatment element. The firing timing determination unit determinesa firing sequence for operating the working chambers of the engine in askip fire manner. The firing sequence is based at least in part on theaftertreatment element temperature data.

Some implementations involve a skip fire engine control system thatdynamically adjusts the firing fraction or firing sequence in responseto a variety of conditions and engine parameters, including oxygensensor data, NO_(x) sensor data, exhaust gas temperature, barometricpressure, ambient humidity, ambient temperature and/or catalyticconverter temperature. In various embodiments, the firing sequence isdetermined on a firing opportunity by firing opportunity basis.

In another aspect a method of operating a lean burn internal combustionengine having a plurality of working chambers during a cold start isdescribed. The method includes deactivating at least one working chambersuch that no air is pumped through the working chamber, obtaining datarelating to a temperature of an element in an aftertreatment system, anddetermining a firing sequence for operating the working chambers of theengine in a skip fire manner. The firing sequence is generated at leastin part based on the aftertreatment temperature data.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1A is a schematic diagram of a representative engine exhaust systemfor an exemplary compression ignition engine.

FIG. 1B is a schematic diagram of an alternative representative engineexhaust system for an exemplary compression ignition engine.

FIG. 2 is a plot of exhaust gas temperature versus engine load for anexemplary compression ignition engine.

FIG. 3 is a skip fire engine controller according to a particularembodiment of the present invention.

FIG. 4 is a plot of exhaust gas temperature versus engine load for anexemplary compression ignition engine operating under skip fire controlwith skipped cylinders being deactivated.

FIG. 5 is a plot of exhaust gas temperature versus engine load for anexemplary compression ignition engine operating under skip fire controlwith skipped cylinders pumping air.

FIG. 6 is a prior art representative plot of an aftertreatment elementtemperature during a cold start-up and over a portion of a drive cycle.

FIG. 7 is a representative plot of an aftertreatment element temperatureduring a cold start-up and over a portion of a drive cycle using skipfire control.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present invention relates to a skip fire engine control system foran internal combustion engine—and particularly, lean burn internalcombustion engines. More specifically, the present invention relates toarrangements and methods for controlling exhaust gas temperature toimprove efficacy of an emission control system. In various embodiments,the firing sequence is determined on a firing opportunity by firingopportunity basis or using a sigma delta, or equivalently a delta sigma,converter. Such a skip fire control system may be defined as dynamicskip fire control.

Skip fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, forexample, a particular cylinder may be fired during one firingopportunity and then may be skipped during the next firing opportunityand then selectively skipped or fired during the next. This iscontrasted with conventional variable displacement engine operation inwhich a fixed set of the cylinders are deactivated during certainlow-load operating conditions. In skip fire operation the firingdecisions may be made dynamically; for example, on a firing opportunityby firing opportunity basis although this is not a requirement.

Skip fire engine control can offer various advantages, includingsubstantial improvements in fuel economy for spark ignition engineswhere pumping losses may be reduced by operating at higher average MAPlevels. Since compression ignition engines do not typically run at lowmanifold pressures, skip fire control does not offer a significantreduction in pumping losses for this type of engine. It does howeverprovide a means to control the engine exhaust gas temperature over awide range of engine operating conditions. In particular, skip firecontrol may be used to increase exhaust gas temperature such that it isgenerally maintained in a range where aftertreatment emission controlsystems can efficiently reduce tailpipe emissions. Skip fire control canoffer a 10% improvement in efficiency in compression ignition engines atlight loads; for example, loads under 1 bar BMEP (Brake Mean EffectivePressure).

FIG. 1A is a schematic diagram of an exemplary lean burn engine andexhaust system. An engine 111 has a number of working chambers orcylinders 114 where combustion occurs. Exhaust gases from the combustionprocess leave the cylinders 114 by way of an exhaust manifold 102. Anexhaust system 103 a includes one or more aftertreatment elements toreduce emission of noxious material into the environment. These elementsmay include a particulate filter 104, an oxidizing catalytic converter106, a reduction agent injection system 108, and a reducing catalyticconverter 113. Collectively these various aftertreatment elements ordevices may be referred to as an aftertreatment system. The particulatefilter 104 removes particulate matter, i.e. soot, which may be presentin the exhaust stream. The oxidizing catalytic converter 106 oxidizesunburned hydrocarbons and carbon monoxide in the exhaust stream. Sincethe engine 111 is generally operating with a lean air/fuel ratio thereis generally adequate oxygen in the exhaust stream to oxidize theseincomplete combustion products. The reduction agent injection system 108introduces a reducing agent, often a mixture of urea and water into thewaste stream. The reducing catalytic converter 113 may use selectivecatalytic reduction (SCR) to reduce nitrous oxides to molecular nitrogenand water. The reducing catalytic converter 113 may use two catalysts. Afirst catalyst 110 to transform urea in the reducing agent 108 toammonia and a second catalyst 112 to transform nitrous oxides andammonia into molecular nitrogen and water. After passing through thereducing catalytic converter 113, the exhaust stream leaves the exhaustsystem 103 a via tailpipe 124 and goes into the environment. The variousaftertreatment elements in the exhaust system 103 a may sufficientlyremove noxious pollutants from the exhaust stream such that it iscompatible with current environmental regulations.

The exhaust system 103 a may additionally include one or more sensors.For example, oxygen sensors 109 a and 109 b may be placed before andafter the oxidizing catalytic converter 106, respectively. A NO_(x)sensor 117 may be situated downstream from the reducing catalyticconverter 113. One or more temperature sensors may also be incorporatedin the exhaust system 103 a. Specifically, there may be a temperaturesensor 107 to monitor the temperature of oxidizing catalytic converter106, a temperature sensor 105 to monitor the temperature of theparticulate filter 104, and a temperature sensor 115 to monitor thetemperature of the reducing catalytic converter 113. Additional sensors(not shown in FIG. 1A or 1B), such as a temperature sensor in theexhaust manifold may be incorporated into the exhaust system.

In order for the aftertreatment elements in an exhaust system toproperly function, they need to operate in a certain elevatedtemperature range. In particular, the catalysts in both the oxidizingcatalytic converter 106 and the reducing catalytic converter 113 need tooperate over a relatively narrow temperature range. A representativeoperating range for the reducing catalyst may be between 200 and 400 C,although other catalysts may have different ranges. The oxidizingcatalyst may have a broader and somewhat higher operating range.Placement of the oxidizing catalyst upstream from the reducing catalystresults in the oxidizing catalyst being generally exposed to highertemperature exhaust gases, since there is less time for the gases tocool in the exhaust system. Generally, aftertreatment elements in theexhaust system may be arranged such that elements with higher operatingtemperature ranges are closer to the engine than the other elements.This allows the first aftertreatment element, for example, theparticulate filter 104 in FIG. 1A, to experience the highest temperatureexhaust stream. The exhaust stream will generally cool as it passesthrough subsequent elements in the exhaust path unless significantenergy is released in any of the aftertreatment devices from exothermicchemical reactions, from an external heat source, oxidization ofuncombusted hydrocarbons, or from some other heat source.

The temperature of an aftertreatment element will generally be close tothat of the exhaust gas passing through it, although in some casesexothermic chemical reactions facilitated by a catalyst may increase itstemperature. Generally exhaust gases will cool as they pass through theexhaust system due to heat transfer from the exhaust system elements andpiping into the environment, although continued oxidization ofuncombusted or partially combusted fuel may increase the exhaust gastemperature. This oxidation may occur both in the exhaust gas stream oron the oxidizing catalyst. The mass of the aftertreatment systemcatalysts are also relatively large compared to the mass flow rate ofexhaust gases through the catalysts, thus it typically takes severalminutes for the catalysts' temperature to equilibrate to that of theexhaust gas flowing through it.

It should be appreciated that the order of the elements in theaftertreatment system may be modified. The arrangement shown in FIG. 1Amay be appropriate for systems where the particulate filter 104 does notrequire an active cleaning or regeneration process. FIG. 1B shows analternative representative exhaust system 103 b. A difference betweenthis system and the exhaust system 103 a shown in FIG. 1A is the orderof the various aftertreatment elements in the exhaust stream. In FIG. 1Bthe oxidizing catalytic converter 106 is placed upstream of theparticulate filter 104. This arrangement may be advantageous when theparticulate filter 104 needs to be regularly cleaned by an activeprocess that raises its temperature to around 500 C to 600 C to burn outaccumulated soot on the particulate filter 104. The active cleaningprocess may include introducing uncombusted hydrocarbons into theexhaust stream and converting them into heat by oxidizing them in theoxidizing catalytic converter 106. By positioning the oxidizingcatalytic converter 106 upstream from the particulate filter 104, theparticulate filter temperature may be actively controlled during thecleaning process. Alternatively, the particulate filter 104 may besituated downstream of the reducing catalytic converter 113. The orderof the aftertreatment elements may vary depending on their operatingtemperature ranges and maximum allowable temperatures.

Various other features and elements not shown in FIGS. 1A and 1B may beincorporated into the exhaust system. Such elements may include, but arenot limited to, an exhaust gas recirculation system (EGR), a turbine topower a turbocharger, and a waste gate to control exhaust gas flowthrough the turbine.

FIG. 2 shows exhaust gas temperature at the exhaust manifold (102 inFIGS. 1A and 1B) versus engine operating load for a representativeboosted, compression ignition engine operating at 1250 rpm. The curve280 represents the exhaust gas temperature as a function of engine loadexpressed in Brake Mean Effective Pressure (BMEP) for a case where allengine cylinders are firing under substantially the same conditions. Theengine output is generally controlled by varying the amount of injectedfuel, although other engine parameters such as fuel injection timing andexhaust gas recirculation may impact engine output. The highest outputload is associated with a stoichiometric air fuel ratio, wheresubstantially all oxygen in the air charge is consumed duringcombustion. Lower output loads correspond to leaner air/fuel ratios andchanging the timing of the fuel injection. Also shown in FIG. 2 asshaded area 270 is an operating temperature range for the engine exhausttemperature that enables efficient aftertreatment operation. Theeffective temperature range may be limited by the SCR catalyst in thereducing catalytic converter 113, which often has a more limitedoperating temperature range than oxidizing catalysts and particulatefilters. The shaded area 270 depicted in FIG. 2 has an operating rangeof about 200 C, which is a typical value. The SCR catalyst needs to bemaintained within this temperature range for the aftertreatment systemto efficiently remove NO_(x) from the exhaust stream. In some cases ifthe SCR catalyst is within its operating temperature range, then theother aftertreatment elements are within their respective operatingranges. In this case the temperature operating range of the SCR catalystrepresents the temperature operating range of the entire aftertreatmentsystem. If other aftertreatment elements in the exhaust system havedifferent or narrower operating temperature ranges the operatingtemperature range 270 depicted in FIG. 2 may need to be modified.

It should be appreciated that the operating range 270 depicted in FIG. 2is not necessarily the operating temperature range of the catalyst, butis the temperature range of exhaust gases in the exhaust manifold thatresult in the catalyst being in its operating temperature range. Forexample, the SCR catalyst operating temperature range may be 200 to 400C and the exhaust gases may cool by 25 C prior to reaching the catalyst.Thus the shaded area 270 represents an exhaust gas temperature range ofapproximately 225 to 425 C. Different engine operating points and enginedesigns may have different steady-state offsets between the exhaust gastemperature and temperature of the SCR catalyst or other catalysts inthe aftertreatment system. In some cases the exhaust gas temperaturesmay rise in the exhaust system due to exothermic chemical reactions. Thevalues given above for the catalyst temperature operating range andoffset temperature between the catalyst and exhaust manifold gastemperature are representative only and should not be construed aslimiting the scope of the present invention.

Inspection of FIG. 2 indicates that only over about half of the engineoperating range do exhaust gas temperatures fall within an acceptablerange for effective NO_(x) removal. Advantageously, skip fire enginecontrol may be used to control exhaust gas temperature over most of anengine operating load range experienced in a typical driving cycle sothat the aftertreatment system temperature remains within its operatingtemperature window. In particular deactivating one or more cylinders,such that no air is pumped through a cylinder during an operating cycle,increases the average temperature of the exhaust gases exiting theengine. A cylinder may be deactivated by deactivating the cylinderintake(s) valves, the cylinder exhaust valve(s) or both intake andexhaust valves(s). Effectively shutting off a cylinder results in lessair to dilute the hot exhaust gases generated by combustion. Inparticular skip fire control may be used to raise the temperature of theexhaust stream under low load conditions.

Referring initially to FIG. 3, a skip fire engine controller 200 inaccordance with one embodiment of the present invention will bedescribed. The engine controller 200 includes a firing fractioncalculator 206, a firing timing determination unit 204, a power trainparameter adjusting module 216, a firing control unit 240 and anaftertreatment monitor 202. The firing control unit 240 receives inputfrom the firing timing determination unit 204, the aftertreatmentmonitor 202, and the power train parameter adjusting module 216 andmanages operation of the working chambers of an engine based on thatinput.

The aftertreatment monitor 202 represents any suitable module, mechanismand/or sensor(s) that obtain data relating to a temperature of anaftertreatment element. It may correspond to the temperature of reducingcatalytic converter 113, oxidizing catalytic converter 106, orparticulate filter 104 (see in FIGS. 1A and 1B). If the reducingcatalytic converter has the narrowest operating range of anyaftertreatment element, only data representative of its temperature maybe used. In various embodiments, for example, the aftertreatment monitor202 may include oxygen sensor data from oxygen sensors 109 a and/or 109b. Aftertreatment monitor may also include measurements from NO_(x)sensors placed before and after the reducing catalytic converter 113.Aftertreatment monitor 202 may also include such inputs as ambient airtemperature, exhaust gas temperature in the exhaust manifold, barometricpressure, ambient humidity and/or engine coolant temperature.Temperature data may be obtained using one or more sensors, such astemperature sensors 105, 107 and 115. In some embodiments, the enginecontroller 200 and the aftertreatment monitor 202 do not require adirect measurement or sensing of the temperature of an aftertreatmentelement. Instead, an algorithm using one or more inputs, such as acatalytic converter temperature model, may be used to determine theaftertreatment element or system temperature. The model is based on oneor more of the above parameters (e.g., oxygen sensor data, NO_(x) sensordata, exhaust gas temperature, ambient temperature, barometric pressure,ambient humidity, etc.) that are representative or related to acatalytic converter temperature. In some embodiments a combination of atemperature model may be combined with measured temperature values todetermine the temperature data. In particular, the model may be used topredict aftertreatment element temperature in transient situations, likeengine startup or transitions between engine loads. A feed forward basedcontrol system may be used to control the temperature of anaftertreatment element in such cases. In still other embodiments, theaftertreatment monitor 202 directly estimates or senses the catalyticconverter temperatures or temperatures of other elements in theaftertreatment system. The aftertreatment monitor 202 transmits theaftertreatment system temperature data to the power train parameteradjusting module 216, the firing timing determination unit 204, thefiring control unit 240 and/or the firing fraction calculator 206.

In addition to the aftertreatment monitor temperature data, the firingfraction calculator 206 receives input signal 210 that is indicative ofa desired torque or other control signal. The signal 210 may be receivedor derived from an accelerator pedal position sensor (APP) or othersuitable sources, such as a cruise controller, a torque controller, etc.

Based on the above inputs, the firing fraction calculator 206 isarranged to determine a skip fire firing fraction (i.e., commandedfiring fraction 223). The firing fraction is indicative of thepercentage of firings under the current (or directed) operatingconditions that are required to deliver the desired output andaftertreatment element temperature. Under some conditions, the firingfraction may be determined based on the percentage of optimized firingsthat are required to deliver the desired output and aftertreatmentelement temperature (e.g., when the working chambers are firing at anoperating point substantially optimized for fuel efficiency). It shouldbe appreciated that a firing fraction may be conveyed or represented ina wide variety of ways. For example, the firing fraction may take theform of a firing pattern, sequence or any other firing characteristicthat involves or inherently conveys the aforementioned percentage offirings.

The firing fraction calculator 206 takes into account a wide variety ofparameters that might affect or help indicate the aftertreatment elementtemperature. That is, the firing fraction is determined at least partlybased on the aftertreatment element temperature data received from theaftertreatment monitor 202. In some approaches, the firing fraction isbased on direct measurement of the aftertreatment element. Additionally,other information may be used to determine the firing fraction, forexample, oxygen sensor data, NO_(x) sensor data, ambient airtemperature, exhaust gas temperature, catalyst temperature, barometricpressure, ambient humidity, engine coolant temperature, etc. In variousembodiments, as these parameters change with the passage of time, thefiring fraction may be dynamically adjusted in response to the changes.

The method used to generate the firing fraction may vary widely,depending on the needs of a particular application. In one particularapproach, the firing fraction is generated at least partly as a functionof time. That is, a preliminary firing fraction value is generated thatis adjusted in a predetermined manner depending on the amount of timethat has passed since engine startup. The preliminary value may then beadjusted further using an algorithm based on any of the aboveparameters, such as ambient air temperature, exhaust gas temperature,catalyst temperature, NO_(x) sensor data, and/or oxygen sensor data. Invarious embodiments, some firing fractions are known to causeundesirable noise, vibration and harshness (NVH) in particular vehicleor engine designs, and such firing fractions may be adjusted or avoided.In still other embodiments, a firing fraction is selected based onaftertreatment element temperature data from a predefined library offiring fractions that have acceptable NVH characteristics. Theaftertreatment element temperature data may be obtained from anaftertreatment element temperature model or may be a sensedaftertreatment element temperature.

In the illustrated embodiment, a power train parameter adjusting module216 is provided that cooperates with the firing fraction calculator 206.The power train parameter adjusting module 216 directs the firingcontrol unit 240 to set selected power train parameters appropriately toinsure that the actual engine output substantially equals the requestedengine output at the commanded firing fraction. By way of example, thepower train parameter adjusting module 216 may be responsible fordetermining the desired fueling level, number of fuel injection events,fueling injection timing, exhaust gas recirculation (EGR), and/or otherengine settings that are desirable to help ensure that the actual engineoutput matches the requested engine output. Of course, in otherembodiments, the power train parameter adjusting module 216 may bearranged to directly control various engine settings.

In some implementations of the present invention, the power trainparameter adjusting module 216 is arranged to shift skipped workingchambers between different modes of operation. As previously noted, skipfire engine operation involves firing one or more selected workingcycles of selected working chambers and skipping others. In a first modeof operation, the skipped working chambers are deactivated duringskipped working cycles—i.e., for the duration of the correspondingworking cycle, very little or no air is passed through the correspondingworking chamber. This mode is achieved by deactivating the intake and/orexhaust valves that allow air ingress and egress into the workingchamber. If both intake and exhaust valves are closed, gases are trappedin the working chamber effectively forming a pneumatic spring.

In a second mode of operation, the intake and exhaust valves for theskipped working chamber are not sealed during the corresponding workingcycle and air is allowed to flow through the working chamber. In thismode of operation no combustion takes place in the skipped workingchamber and the air pumped through the skipped working chamber isdelivered to the exhaust system. This has the effect of diluting theexhaust stream and lowering its temperature. It also introduces excessoxygen into the exhaust stream.

In a third mode of operation, the intake and exhaust valves of a skippedworking chamber open and fuel is injected into the cylinder late in thepower stroke. The result is uncombusted or only slightly combusted fuelin the exhaust stream delivered by the skipped working chambers. Theuncombusted hydrocarbons enter the oxidizing catalytic converter andreact exothermically with the air from the skipped working chambers.This reaction helps to heat the oxidizing catalytic converter. Such anapproach can be particularly useful during an engine start-up period inwhich the oxidizing catalytic converter needs to be rapidly heated inorder to minimize the emission of pollutants. In other embodiments, theuncombusted hydrocarbons may be useful to clean a particulate filter 104by raising its temperature to burn off accumulated soot. Whiledeliberating introducing hydrocarbons into the exhaust system may beuseful in some situations, this practice should be generally avoided orminimized, since it reduces fuel economy.

It should be appreciated that the skipped cylinders can be operated inany of the three modes of operation, i.e. deactivated, operating valveswithout fuel injection, or injecting fuel in a manner that results inlittle or no combustion. That is on some working cycles a skippedcylinder may be operated with disabled valves and on a subsequent cyclewith operating valves and on a following cycle with disabled valves.Whether a cylinder is skipped or fired is also controlled in a dynamicmanner. This level of control allows optimization of the amount of air,oxygen, and uncombusted fuel delivered into the exhaust system by theskipped cylinders. The fired cylinders generally produce hot exhaustgases containing some residual oxygen, since the cylinders are generallyrunning lean, as well as some residual level of unburned hydrocarbons.

Controlling emissions during engine start-up is technically challengingbecause the various aftertreatment elements have not reached theiroperating temperatures. Initially from a cold start all engine andexhaust components are cold. It may be desirable to start the engine ata relatively low firing fraction, with firing cylinders at a nominallystoichiometric air/fuel ratio, and keep all the skipped cylinders inmode one to avoid pumping any air into the oxidizing catalyticconverter. Once the oxidizing catalytic converter temperature hasstarted to rise, oxygen may be delivered to the catalytic converter byoperating at least some of the skipped cylinders in the second or thirdmode. Unburned hydrocarbons may simultaneously be delivered to theoxidizing catalytic converter by running the firing cylinders at a richair/fuel ratio or via late fuel injection through the skipped cylinders,i.e. mode three operation. The oxygen and unburned hydrocarbons may thenexothermically react in the oxidizing catalyst converter to more rapidlyincrease its temperature. This reaction may occur only once theoxidizing catalyst converter is at or above hydrocarbon light-offtemperature and thus it may be desirable to only introduce oxygen andunburned hydrocarbons to the catalytic converter once it has reachedthat temperature. Once the oxidizing catalytic converter has reached itsoperational temperature all the skipped cylinders may be operated inmode three, late fuel injection through the skipped cylinders, to raisethe exhaust gas temperature. It should be appreciated that heating theoxidizing catalyst by supplying it with unburned fuel and oxygen willalso heat other aftertreatment elements downstream in the exhaust systemfrom the oxidizing catalyst. These aftertreatment elements may includethe reducing catalyst and/or particulate filter.

In various implementations, the power train parameter adjusting module216 is arranged to cause the engine to shift between the three modes ofoperation based on the aftertreatment element temperature data and/orother engine operating parameters. For example, in some approaches, ifthe engine controller determines that the temperature of theaftertreatment element is below its effective operating temperaturerange, but above the light-off temperature, (e.g., during a cold startor under extended low load conditions), the power train parameteradjusting module may utilize the third mode of operation (e.g., a skipfire engine operation that involves the delivery of unburnedhydrocarbons to the oxidizing catalytic converter). This mode ofoperation can help expedite the heating of the oxidizing catalyst to adesired operating temperature. If, however, the engine controllerdetermines that the temperature of the oxidizing catalyst is high enoughor has reached an effective operating temperature range temperature, thepower train parameter adjusting module will shift to the first mode ofoperation (e.g., skip fire engine operation involving delivery of a leanair fuel mixture to the fired working chambers and deactivation of theskipped working chambers.)

There also may be situations in which the catalyst temperature is toohigh and cooling is required. For example, compression ignition enginesare typically coupled with an aftertreatment emission control elementthat operates in a somewhat narrower band of operating temperatures thanspark ignition engines. In some cases, the temperature of theaftertreatment element may exceed this band. It is desirable to avoidsuch situations, since excessive temperatures can damage or impair theperformance of the aftertreatment element. Accordingly, in someembodiments, the engine controller makes a determination as to whetherthe aftertreatment element has exceeded a particular thresholdtemperature. If that is the case outside air may be injected into theexhaust system prior to any aftertreatment element whose temperatureexceeds its operational temperature range. The extra air that flowsthrough the exhaust system will help cool the aftertreatment emissioncontrol element. Once the engine controller determines that thetemperature of the aftertreatment element is within a desired operatingtemperature band, the outside air injection may be terminated.

It should be appreciated that in some embodiments, different modes maybe applied to different working cycles. In other words, during aselected working cycle of a particular working chamber, the workingchamber may be operated in a second mode while in the very next workingcycle, the corresponding working chamber will be operated in a firstmode. In other words, in one working cycle, the skipped working chambermay allow for the passage of air, while in the very next firingopportunity that involves a skipped working chamber, the working chamberis deactivated and sealed. Changes in the delivery of air-fuel mixturesand the operation of the working chamber valves can change dynamicallyfrom one working cycle to the next and from one working chamber to thenext in response to the exhaust system temperature data and/or a varietyof engine operating parameters.

The firing timing determination unit 204 receives input from the firingfraction calculator 206 and/or the power train parameter adjustingmodule 216 and is arranged to issue a sequence of firing commands (e.g.,drive pulse signal 213) that cause the engine to deliver the percentageof firings dictated by the commanded firing fraction 223. The firingtiming determination unit 204 may take a wide variety of differentforms. For example, in some embodiments, the firing timing determinationunit 204 may utilize various types of lookup tables to implement thedesired control algorithms. In other embodiments, a sigma deltaconverter or other mechanisms are used. The sequence of firing commands(sometimes referred to as a drive pulse signal 213) outputted by thefiring timing determination unit 204 may be passed to a firing controlunit 240 which orchestrates the actual firings.

Some implementations involve selective firing of particular workingchambers and not others. For example, during an engine startup periodfrom a cold start, the engine controller may fire only a particularsubset of working chambers that are physically closer to anaftertreatment element in the exhaust system, i.e have a shorter exhauststream path to the aftertreatment element. Since exhaust from thoseworking chambers has a shorter path to travel, the exhaust loses lessthermal energy and can help heat the aftertreatment elements morequickly and efficiently. At least one working chamber may be deactivatedsuch that no air is pumped through the working chamber raising thetemperature of the exhaust gases for a number of working cycles. This atleast one deactivated working chamber may be the working chamberpositioned farthest from the aftertreatment elements being heated.

The engine controller 200, firing fraction calculator 206, the powertrain parameter adjusting module 216, and the firing timingdetermination unit 204 may take a wide variety of different forms andfunctionalities. For example, the various modules illustrated in FIG. 3may be incorporated into fewer components or have their featuresperformed by a larger number of modules. Additional features and modulesmay be added to the engine controller. By way of example, some suitablefiring fraction calculators, firing timing determination units, powertrain parameter adjusting modules and other associated modules aredescribed in co-assigned U.S. Pat. Nos. 7,954,474; 7,886,715; 7,849,835;7,577,511; 8,099,224; 8,131,445; 8,131,447; 9,086,020; and 9,120,478:U.S. patent application Ser. Nos. 13/774,134; 13/963,686; 13/953,615;13/886,107; 13/963,759; 13/963,819; 13/961,701; 13/843,567; 13/794,157;13/842,234; 13/004,839, 13/654,244 and 13/004,844, each of which isincorporated herein by reference in its entirety for all purposes. Anyof the features, modules and operations described in the above patentdocuments may be added to the controller 200. In various alternativeimplementations, these functional blocks may be accomplishedalgorithmically using a microprocessor, ECU or other computation device,using analog or digital components, using programmable logic, usingcombinations of the foregoing and/or in any other suitable manner.

FIG. 4 shows exhaust manifold gas temperature versus engine operatingload for a representative boosted, compression ignition engine operatingat 1250 rpm with skip fire control. The engine operating load isexpressed in BMEP relative to the total engine displacement with allcylinders operating. A plurality of operating curves 410 a thru 410 jare shown in FIG. 4. These correspond to operating the engine withdifferent firing fractions with deactivated cylinders, operational modeone described above. The left most curve 410 a corresponds to the lowestfiring fraction and the right most curve 410 j corresponds to thehighest firing fraction, i.e. a firing fraction of 1, all cylindersfiring. Intermediate curves 410 b thru 410 i correspond to successivehigher firing fractions. Curve 410 j, corresponding to a firing fractionof one, is identical to the corresponding portion of curve 280 shown inFIG. 2. The firing fractions chosen may correspond to firing fractionsproviding acceptable NVH (noise, vibration, and harshness)characteristics as described in co-assigned U.S. Pat. No. 9,086,020 andpending U.S. patent application Ser. Nos. 13/963,686 and 14/638,908.Also shown in FIG. 4 is the shaded region 270 which depicts the exhaustmanifold gas temperature required to elevate an aftertreatment elementin the exhaust system into its operational temperature range. Thisregion is identical to the shaded region 270 shown in FIG. 2.

Within the allowed operating region the required engine output can begenerated by operating on one of the firing fractions denoted in curves410 a thru 410 j while maintaining the engine exhaust gas temperaturewithin the required temperature limits. In some cases several firingfractions may deliver acceptable engine output and exhaust gastemperature. In these cases the firing fraction providing the most fuelefficient operation may be selected by the firing fraction calculator206 (FIG. 3) to operate the engine. Inspection of FIG. 4 shows that theallowed steady-state operating range using skip fire control is muchlarger than the corresponding range without such control as shown inFIG. 2. Similar operating curves 410 a thru 410 j and exhaust manifoldgas temperature ranges 470 can be generated for other engine speeds.Generally operation in skip fire control with varying firing fractionswill allow steady-state operation over a wide range of engine loads.

As shown in FIG. 4 there is a range of high load conditions, aboveapproximately 9.5 bar BMEP, where the engine cannot operate insteady-state and maintain the element in the aftertreatment system inits desired temperature range. These high loads require operation on allcylinders and thus no cylinders are skipped, i.e. a firing fraction ofone. Under these conditions the aftertreatment system temperature may bereduced by injecting outside air into the exhaust system as previouslydescribed. Also, under typical driving cycles an engine seldom operatesin this high load region over extended periods of time. Operation for ashort time period in this high load regime, such as when passing orgoing up a steep hill, will generally not result in the aftertreatmentsystem exceeding its operating temperature range due to the thermalinertia of the aftertreatment system. In these cases no outside coolingneed be applied to the aftertreatment system, since it will not exceedits operating temperature range.

Operation of a compression ignition internal combustion with skip firecontrol and deactivating the skipped cylinders allows maintaining a highexhaust gas temperature over a large engine operating range. Highexhaust gas temperatures are generally advantageous for a particulatefilter (104 in FIGS. 1A and 1B), which may be part of an aftertreatmentsystem. Maintaining the particulate filter 104 at a high temperaturewill encourage oxidization of soot particles trapped in the filter.

Some particulate filters 104 require their temperature to be raised,periodically, to around 500 C to 600 C to remove accumulated soot on thefilter in order for the filter to function again. This activetemperature management process is very fuel consuming. Even though theclean-out/regeneration process has to occur every 200 to 400 miles,depending on size of filter, the overall penalty on fuel economy can besignificant. Skip fire operation may reduce or eliminate the need forregeneration where the particulate filter 104 is heated to fully oxidizesoot trapped in the filter thus cleaning the filter. With skip fireoperation the particulate filter can generally operate at a highertemperature, which reduces the soot build up rate lengthening the periodbetween cleaning cycles. In some cases, skip fire control may be used totemporarily deliberately raise the exhaust stream temperature to cleanthe particulate filter 104 in an active regeneration process. Such acleaning method will be more fuel efficient than cleaning methods thatrely on introducing uncombusted hydrocarbons into the exhaust stream.

FIG. 5 shows exhaust manifold gas temperature versus engine operatingload for a representative boosted, compression ignition engine operatingat 1250 rpm with skip fire control. FIG. 5 is similar to FIG. 4 exceptthe skipped cylinders are not deactivated, they are pumping air withoutany added fuel (mode 2). The various curves 510 a-510 j representoperation at different firing fractions. The numeric designators 510 athru 510 j represent the same firing fractions as shown in FIG. 4. Curve510 j, which corresponds to operation with a firing fraction of one, isidentical to curve 410 j and curve 280. This is operational mode two asdescribed above. Shaded region 270 is identical to that shown in FIG. 2and FIG. 4 and represents the exhaust gas temperature required to heatan element in the aftertreatment system into its operational temperaturerange. Inspection of FIG. 5 shows that operation in mode two withoutadding fuel to the skipped cylinders does little to expand the range ofallowable steady-state operating conditions relative to all cylindersfiring (shown in FIG. 2). Operation of some skipped cylinders in mode 2,pumping, and some skipped cylinders in mode 1, deactivated, may beuseful to control exhaust gas temperature in some cases.

Two operating areas where skip fire control is particularly useful areduring start-up and at light loads where a compression ignition engineusually runs very lean. This is because the fuel flow is very low asresult of the light load. In most older compression ignition engines theair flow cannot be further reduced since these engines generally have nothrottle. Therefore, the exhaust temperature can sometimes be too lowfor effective NO_(x) conversion in the catalyst. Some prior art solutionhave used hydrocarbon injection into the exhaust system to generateadditional heat in the exhaust system maintaining one or moreaftertreatment elements in their desired operational temperature range.This control method sacrifices fuel economy. Use of skip fire controlmay obviate—or at least significantly reduce—the need for thishydrocarbon injection.

FIG. 6 shows the temperature of an aftertreatment element 606 over coldstart-up and a portion of a drive cycle of a prior art compressionignition engine. The figure also shows the light-off temperature 602,the temperature at which some hydrocarbons in the exhaust stream willself-ignite, and the lower boundary of the effective operating range ofthe aftertreatment element 604. In this figure the light-off temperatureis shown as 150 C and the lower aftertreatment operating range as 200 C;however, these should be treated as only representative values and inpractice they may be larger or smaller.

The drive cycle begins with the aftertreatment element at ambienttemperature, assumed to be 20 C. The aftertreatment element reacheslight-off temperature at time t₁. It is only after this time thatinjecting hydrocarbons into the exhaust stream can raise the temperatureof an aftertreatment element. The aftertreatment element temperaturecontinues to rise until time t₂ where it reaches its effective operatingrange. Prior to time t₂ the aftertreatment element is ineffective atremoving pollutants from the exhaust stream. The aftertreatment elementremains effective at removing pollutants until time t₃, which representsan extended low load portion of the drive cycle. For the period betweent₃ and t₄ the aftertreatment element is below its operating range and isineffective at removing pollutants.

To reduce emissions, it is desirable to reduce the start-up time untilthe aftertreatment element reaches its operating temperature and reduceor eliminate the aftertreatment element falling below its operatingtemperature during low load conditions. FIG. 7 shows a representativedrive cycle using the current invention. The prior art aftertreatmentelement temperature is shown as the dotted line 606, which is identicalto the aftertreatment temperature shown in FIG. 6. The light-offtemperature 602 and lower limit of the aftertreatment element operatingtemperature 604 are as in FIG. 6. Curve 608 depicts the aftertreatmentelement temperature. The time for the aftertreatment element to reachits operating temperature has been reduced from t₂ to t_(2′). Also, theaftertreatment element is maintained within its effective operatingrange in the extended low load period between t₃ and t₄. The enginecontroller may maintain the aftertreatment element temperature somewhatabove its minimum operating temperature range to maintain a buffer abovethis value. Maintaining a buffer helps to prevent the aftertreatmentelement temperature from falling below its operating range should theengine load be further reduced. Engine emissions over the drive cycleare thus lower when using the current invention shown in FIG. 7 ascompared to the prior art shown in FIG. 6.

The invention has been described primarily in the context of controllingthe firing of 4-stroke, compression ignition, piston engines suitablefor use in motor vehicles. The compression ignition may use a stratifiedfuel charge, a homogeneous fuel charge, partial homogeneous charge, orsome other type of fuel charge. However, it should be appreciated thatthe described skip fire approaches are very well suited for use in awide variety of internal combustion engines. These include engines forvirtually any type of vehicle—including cars, trucks, boats,construction equipment, aircraft, motorcycles, scooters, etc.; andvirtually any other application that involves the firing of workingchambers and utilizes an internal combustion engine.

In some preferred embodiments, the firing timing determination moduleutilizes sigma delta conversion. Although it is believed that sigmadelta converters are very well suited for use in this application, itshould be appreciated that the converters may employ a wide variety ofmodulation schemes. For example, pulse width modulation, pulse heightmodulation, CDMA oriented modulation or other modulation schemes may beused to deliver the drive pulse signal. Some of the describedembodiments utilize first order converters. However, in otherembodiments higher order converters or a library of predetermined firingsequences may be used.

In other embodiments of the invention, intake and exhaust valve controlmay be more complex than simple binary control, i.e. open or closed.Variable lift valves may be used and/or the valve opening/closing timingmay be adjusted by a cam phaser. These actuators allow limited controlof cylinder MAC without use of a throttle and its associated pumpinglosses. Advantageously adjustment of the cylinder MAC allows control ofthe fuel/air stoichiometry for a fixed fuel charge. The combustionconditions may then be optimized for improved fuel efficiency or toprovide desired conditions, i.e. oxygen level, temperature, etc., in thecombustion exhaust gases.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. For example, the drawings and the embodiments sometimesdescribe specific arrangements, operational steps, and controlmechanisms. It should be appreciated that these mechanisms and steps maybe modified as appropriate to suit the needs of different applications.For example, the order of the various aftertreatment emission controlelements in the exhaust path shown in FIGS. 1A and 1B may be altered.Additional aftertreatment devices may be use and the functionality ofindividual elements combined into a single element. The methods ofperforming the oxidization and reduction steps may be altered; forexample, a lean NO_(x) trap may be used in place of the SCR catalyst.Using a NO_(x) adsorber/NO_(x) trap for a lean burn engine requires theengine to run slightly rich periodically, such as every minute or two,to purge the NO_(x) from the adsorber, regenerating the NO_(x) trap.Since compression ignition engines typically operate under very leanconditions, especially at light loads, to operate the engine under richcondition to purge the NO_(x) trap requires a significant reduction inair flow through the engine, which normally requires throttling theengine air flow. The regeneration process is also a very fuel consuming.It would be very advantageous to use skip fire control to purge a NO_(x)trap compared to traditional methods of purging NO_(x) traps. Use ofskip fire control may obviate the requirement to have a throttle in theengine, reducing engine cost and complexity. In other embodiments, useof skip fire control may be used in concert with an engine throttle tocontrol exhaust gas temperature.

Additionally, while the invention has been generally described in termsof a compression ignition engine, it may also be used in spark ignition,spark ignition assisted, or glow plug ignition assisted engines. Inparticular, the invention is applicable to lean burn spark ignitionengines. These engines have some of the attributes of compressionignition engines, such as oxygen in the exhaust stream, so they cannotgenerally use a conventional 3-way catalyst based aftertreatment system.In some embodiments not all of the cylinders in an engine need becapable of deactivation. This may reduce costs relative to an enginehaving all cylinders capable of deactivation. In some embodiments, oneor more of the described operations are reordered, replaced, modified orremoved. Therefore, the present embodiments should be consideredillustrative and not restrictive and the invention is not to be limitedto the details given herein.

What is claimed is:
 1. An engine controller for operating a lean burninternal combustion engine including one or more working chambers, theengine controller configured for: selectively firing or skipping one ormore working cycles of the one or more working chambers, wherein one ofthe one or more working chambers may be fired during one engine cycleand then skipped during the next engine cycle and selectively skipped orfired during the next engine cycle; and injecting fuel during at leastsome of the skipped working cycles such that at least some of theinjected fuel is introduced into an exhaust stream fluidly coupled tothe one or more working chambers; wherein injecting fuel comprisesinitiating injection of the fuel at a location in a power stroke of theskipped working cycle so that little or no combustion occurs resultingin uncombusted fuel in the exhaust stream.
 2. The controller as recitedin claim 1, the controller further configured for: for a given skippedworking cycle, operating a corresponding working chamber by: opening anintake valve associated with the working chamber and inducting air intothe working chamber during an intake stroke; closing the intake valveand an exhaust valve associated with the working chamber and compressingthe inducted air during a compression stroke; initiating the powerstroke and timing injection of the fuel late in the power stroke so asto inhibit combustion of the fuel; and opening the exhaust valve so thatinducted air and the injected fuel is exhausted into the exhaust streamduring an exhaust stroke.
 3. The controller as recited in claim 1, thecontroller further configured for: controlling an operating temperatureof an aftertreatment element fluidly coupled to the lean burn internalcombustion engine by selectively firing or skipping the one or moreworking chambers.
 4. The controller as recited in claim 3, whereincontrolling the operating temperature of the aftertreatment elementfurther comprises operating the lean burn internal combustion engine inone of three modes for skipped working cycles, the three modesincluding: (i) a first mode wherein the skipped working cycle(s) is/aredeactivated and no air is allowed to flow through the skipped workingchamber(s); (ii) a second mode wherein air is allowed to flow throughthe skipped working chamber(s) of skipped working cycle(s) and no fuelis injected; and (iii) a third mode wherein the injected fuel isintroduced into the exhaust stream and allowed to pass through theworking chamber(s) of skipped working cycle(s).
 5. The controller asrecited in claim 4, the controller further configured for: selecting thesecond mode to reduce the operating temperature of the aftertreatmentelement; and selecting either the first mode or the third mode toincrease the operating temperature of the aftertreatment element.
 6. Thecontroller as recited in claim 1, the controller further configured for:determining an operating temperature of an oxidizing catalytic converterincluded in an exhaust system fluidly coupled to receive the exhauststream; and determining to either fire or skip the one or more workingchambers at least partially based on if the determined operatingtemperature of the oxidizing catalytic converter is below a thresholdtemperature; whereby inducted air and the fuel passed through the one ormore skipped working chambers exothermally react within the exhauststream to heat the oxidizing catalytic converter.
 7. The controller asrecited in claim 1, the controller further configured for: operating thelean burn internal combustion engine following a cold start byexothermally reacting the fuel and inducted air in the exhaust stream torapidly increase a temperature of an oxidizing catalytic converter suchthat the temperature of the oxidizing catalytic converter rises fasterthan if the fuel was not present in the exhaust stream.
 8. Thecontroller as recited in claim 1, the controller further configured for:determining an operating temperature of a particulate filter included inan exhaust system fluidly coupled to receive the exhaust stream;determining to either fire or skip the one or more working chambers atleast partially based on if the determined operating temperature of theparticulate filter is below a threshold temperature; and burning off thefuel in the exhaust system that passed through the one or more skippedworking cycles of the one or more working chambers, the burning off ofthe fuel in the exhaust system acting to burn off accumulated soot inthe particulate filter.
 9. The controller as recited in claim 8, whereinburning off the fuel in the exhaust system further comprises heating theparticulate filter to a temperature in the range of approximately 500°C. to 600° C.
 10. The controller as recited in claim 1, the controllerfurther configured for: operating the lean burn internal combustionengine in a pumping mode for other skipped working cycles, air flow isenabled to pass through the one or more working chambers into theexhaust stream, the air flow acting to reduce a temperature of anaftertreatment element within the exhaust stream.
 11. The controller asrecited in claim 10, wherein air is allowed to flow through each of theone or more working chambers by opening an intake valve and an exhaustvalve associated with each working chamber respectively.
 12. Thecontroller as recited in claim 1, the controller further configured for:operating a skipped working cycle in a deactivation mode wherein intakeand exhaust valves associated with the working chamber of the skippedworking cycle are closed and substantially no air passes through the oneor more working chambers into the exhaust stream; wherein a lack of airpassing through the one or more working chambers into the exhaust streamacts to increase a temperature of the exhaust stream.
 13. The controlleras recited in claim 1, the controller further configured for: defining afiring fraction for the one of more working chambers, wherein thecontroller considers one or more of the following when defining thefiring fraction: aftertreatment element operating temperature; oxygensensor data; NO_(x) sensor data; ambient air temperature; exhaust gastemperature; barometric pressure; or catalyst temperature.
 14. Thecontroller of claim 1, further comprising operating a skip fire enginecontroller to decide to either fire or skip each of the one or moreworking cycles of the lean burn internal combustion engine on a firingopportunity by firing opportunity basis.
 15. The controller as recitedin claim 1, wherein the lean burn internal combustion engine is one ofthe following: a Diesel engine; a boosted engine; or a turbo chargedengine.
 16. The controller as recited in claim 1, wherein an output ofthe lean burn internal combustion engine is controlled at least in partby an exhaust gas recirculation system.
 17. The controller as recited inclaim 1, wherein at least one of the working chambers of the internalcombustion engine is capable of deactivation and at least one of theworking chambers of the internal combustion engine is incapable ofdeactivation.
 18. The controller as recited in claim 17, wherein the atleast one working chamber capable of deactivation is deactivated bydeactivating an intake valve, deactivating an exhaust valve, ordeactivating both the intake and the exhaust valve during a skippedworking cycle.
 19. The controller as recited in claim 1, wherein atleast some of the fired working cycles utilize an air/fuel ratio that issufficiently rich to cause unburnt hydrocarbons to be introduced intothe exhaust stream.
 20. The controller as recited in claim 1, wherein atleast some of the fired working cycles introduce partially combustedfuel into the exhaust stream and the partially combusted fuel is furtheroxidized in the exhaust stream or on an oxidizing catalyst.
 21. Anengine controller for operating a lean burn internal combustion engineincluding a plurality of working chambers, the engine controllerconfigured for: selectively firing or skipping one or more workingcycles of the one or more working chambers, wherein one of the one ormore working chambers may be fired during one engine cycle and thenskipped during the next engine cycle and selectively skipped or firedduring the next engine cycle; injecting fuel during at least some of theskipped working cycles such that at least some of the injected fuel isintroduced into an exhaust stream fluidly coupled to the one or moreworking chambers; for a given skipped working cycle, operating acorresponding working chamber by: opening an intake valve associatedwith the working chamber and inducting air into the working chamberduring an intake stroke; closing the intake valve and an exhaust valveassociated with the working chamber and compressing the inducted airduring a compression stroke; initiating a power stroke and injecting thefuel late in the power stroke, the injection of the fuel late in thepower stroke substantially preventing the combustion of the fuel; andopening the exhaust valve so that inducted air and the injected fuel isexhausted into the exhaust stream during an exhaust stroke.
 22. Anengine controller for operating a lean burn internal combustion engineincluding a plurality of working chambers, the engine controllerconfigured for: selectively firing or skipping one or more workingcycles of the one or more working chambers, wherein one of the one ormore working chambers may be fired during one engine cycle and thenskipped during the next engine cycle and selectively skipped or firedduring the next engine cycle; injecting fuel during at least some of theskipped working cycles such that at least some of the injected fuel isintroduced into an exhaust stream fluidly coupled to the one or moreworking chambers; and initiating injection of the fuel at a locationlate in the skipped working cycle so that little or no combustion occursresulting in uncombusted fuel in the exhaust stream.
 23. The controlleras recited in claim 22, wherein initiation of the fuel injection istimed to occur in a power stroke of the skipped working cycle.
 24. Anengine controller for operating a lean burn internal combustion engineincluding a plurality of working chambers, the engine controllerconfigured for: selectively firing or skipping one or more workingcycles of the one or more working chambers, wherein one of the one ormore working chambers may be fired during one engine cycle and thenskipped during the next engine cycle and selectively skipped or firedduring the next engine cycle; and injecting fuel during at least some ofthe skipped working cycles such that at least some of the injected fuelis introduced into an exhaust stream fluidly coupled to the one or moreworking chambers; controlling an operating temperature of anaftertreatment element fluidly coupled to the lean burn internalcombustion engine by operating the lean burn internal combustion enginein one of three modes for skipped working cycles, the three modesincluding: (i) a first mode wherein the skipped working cycle(s) is/aredeactivated and no air is allowed to flow through the skipped workingchamber(s); (ii) a second mode wherein air is allowed to flow throughthe skipped working chamber(s) of skipped working cycle(s) and no fuelis injected; and (iii) a third mode wherein the injected fuel isintroduced into the exhaust stream and allowed to pass through theworking chamber(s) of skipped working cycle(s).